1. Field of the Invention
The present invention relates in general to an improvement of a grinding wheel of segmental type including an inner annular or cylindrical core portion and an outer grinding portion consisting of a plurality of segments which are bonded to the outer circumferential surface of the core portion such that the segments are arranged in the circumferential direction of the core portion.
2. Discussion of Related Art
There is known a segmental type of grinding wheel including an inner annular or cylindrical core portion and an outer grinding portion consisting of a plurality of segments which are bonded to the outer circumferential surface of the core portion such that the segments are arranged in the circumferential direction of the core portion. Each segment has a layer of abrasive grains. In operation, the grinding wheel is rotated about an axis of the cylindrical core portion, with an annular array of the segments being held in contact with a surface of a workpiece, so that the workpiece surface is ground by the abrasive grains of the segments. The grinding wheel takes the form of this segmented grinding wheel wherein the segments are arranged for contact with the workpiece surface to be ground, in most cases where the abrasive layer serving to perform a grinding operation on the workpiece is formed of a so-called “super abrasive” such as diamond abrasives or CBN abrasives, which have a comparatively long service life and are more expensive than ordinary abrasives such as alumina abrasives or silicon carbide abrasives. As one kind of the segmental type grinding wheel, there is known a grinding wheel including a cylindrical core portion and a plurality of segments which have abrasive layers formed of abrasive grains bonded together with a glass bonding or binding agent and which are bonded to the outer circumferential surface of the core portion with a synthetic resin layer interposed therebetween. Since this kind of segmental type grinding wheel using the glass bonding agent permits a grinding operation with a high degree of accuracy and has excellent properties such as high durability, this segmental type grinding wheel is used in various fields of industry, and has been an object of further research and development for further improvement of its grinding performance.
Conventionally, the segmental type grinding wheel is usually operated at a peripheral speed of not higher than 4800 m/min. The grinding wheel is generally rotated under a non-load condition for some length of time, for example, during a warm-up period of a grinding machine prior to an actual grinding operation thereof, or during an interruption of the actual grinding operation. The temperature of the grinding wheel during the non-load operation at a peripheral speed as indicated above is almost equal to or slightly higher than the ambient temperature, for instance, the temperature of a coolant used for cooling a worktable of the grinding machine.
When the grinding wheel is operated at a peripheral speed of 4800 m/min or higher, the temperature of the grinding wheel during the non-load operation is raised to a comparatively high level. The graph of FIG. 1 indicates a relationship between the time (min) of operation of the grinding wheel under the non-load condition at a peripheral speed of 12000 m/min and the temperature (° C.) at the surface of the grinding wheel. As indicated by this graph, the surface temperature of the grinding wheel simply rises with an increase in the time of the non-load operation at that peripheral speed. The surface temperature rises to a comparatively high level of about 70° C. when the non-load operation of the grinding wheel has been performed for about one hour. Generally, the segments of the segmental type grinding wheel are bonded to the core portion with a synthetic resin bonding agent such as an epoxy resin, which has a tendency that the bonding strength decreases with an increase in the temperature of the grinding wheel. To prevent a decrease in the bonding strength of the synthetic resin bonding agent during the non-load operation of the segmental type grinding wheel at a comparatively high peripheral speed, it is a conventional practice to continue a supply of the coolant to the grinding wheel even in its non-load operation, for avoiding an excessive rise of the temperature of the grinding wheel. Thus, it is conventionally required to continue the coolant supply even in a period of time in which the grinding wheel is not engaged in an actual grinding operation on the workpiece.
As one means for assuring a sufficiently high bonding strength of a synthetic resin bonding agent such as an epoxy resin bonding agent of two-liquid mixture type, it is known to harden or cure the synthetic resin bonding agent in an atmosphere having a temperature as high as possible. The synthetic resin bonding agent cured at such a high temperature has a sufficiently high bonding strength even after the temperature of the grinding wheel has been raised to a level close to the curing temperature. On the other hand, however, the segmental type grinding wheel using a synthetic resin bonding agent cured at such a comparatively high temperature suffers from a tendency of easy breakage or cracking of the segments during transportation or storage of the grinding wheel.
The present inventors made an extensive study in an effort to solve the drawbacks of the known segmental type grinding wheel which have been discussed. The study revealed that the tendency of easy breakage or cracking of the segments of the segmental type grinding wheel in which the synthetic resin bonding agent is cured at a comparatively high temperature is caused by a compressive strain induced in the segments due to thermal contraction of the segments, which in turn is caused by a temperature drop of the segments from the curing temperature of the synthetic resin bonding agent. Namely, the inventors arrived at a finding that the segments of the known segmental type grinding wheel tend to suffer from easy breakage or cracking during transportation or storage of the grinding wheel, since the segments are subjected to a considerably large compressive strain due to a temperature drop of the segments from the comparatively high curing temperature of the synthetic resin bonding agent, in the presence of a considerably large difference in the coefficient of thermal expansion (coefficient of linear thermal expansion) between the segments and the core portion.